It is known in the space heating art to provide both heating and cooling to a building interior using a reversible circuit electric heat pump that operates to either pump indoor ambient heat out of the building (during the heat pump's cooling cycle) or, with its refrigerant circuit reversed by operation of a reversible valve in the circuit, pump ambient outdoor air heat into the building (during the heat pump's heating cycle). Particularly in cold northern climates, winter temperatures commonly reach low temperatures that make it difficult if not impossible to wring sufficient heat out of the frigid outside air to sufficiently heat the building interior using only the heat pump. Because of this it has been common practice to add to the heat pump auxiliary heating in the form of electric resistance strip heaters that supplement the refrigerant heating capacity of the heat pump when heating conditions warrant.
While this meets the comfort requirements of the building, it also substantially increases the yearly heating bill due the normally much higher cost of electric resistance heating compared to the refrigerant-based heating provided by a heat pump. As an alternate to this resistance heat add-on technique, various proposals have been made to supplement the refrigerant heating capacity of a reversible heat pump with a fuel-fired supplemental heating source used in place of the heat pump during high heating demand periods when the heat pump cannot provide sufficient heat by itself. However, a common shortcoming of such proposals has been their tendency to lessen the overall resulting efficiency of the heat pump/fuel-fired hybrid heating system to an undesirable degree. Accordingly, a need exists for a heat pump/fuel-fired hybrid heating system that strikes a more desirable balance between efficiency and operating costs, while restricting the user from overriding the heat pump. It is to this need that the present invention is primarily directed.